1. Technical Field
This invention relates to ophthalmic implants and the manipulation of the intraocular pressure of the eye, and, more particularly, to a new and useful method and apparatus for lowering intraocular pressure by draining aqueous humor from the anterior chamber of the eye and by mechanically inhibiting wound healing at the surgical site.
2. Background
Throughout the United States, Europe and most of the first world countries, glaucoma is the most prevalent sight threatening disease and is, on a world wide basis, responsible for approximately ten percent of all blindness. Glaucoma is an ocular disease where ocular fluids build up in the eye and exert tremendous pressure on the optic nerve, slowly causing irreparable damage as a result of glaucomatous optic neuropathy.
Glaucoma is a significant healthcare problem with immediate and long term ramifications, both physical and financial. Glaucoma is the leading cause of blindness in the United States, where over 300,000 new cases are reported each year. In the U.S. more than 95,000 glaucoma patients lose some degree of sight each year due to the disease, with greater than 5,500 experiencing total blindness. As treatment costs on a per year basis are estimated to be $1.5 billion based upon more than two million annual office visits, the socio-economic impact of glaucoma is significant.
The causes of glaucoma are poorly understood; however, vasomotor and emotional instability, hyperopia, and especially heredity are among the recognized predisposing factors. Along with persons predisposed to glaucoma by virtue of family history, individuals at higher risk of developing glaucoma are those 35 years of age or older or those with diabetes or positive glucose tolerance tests. The disease also strikes African-Americans in disproportionate numbers. They are four to five times more likely to develop glaucoma and are up to six times more apt to suffer a complete loss of sight.
The increased intraocular pressure incident to glaucoma is related to an imbalance between production and outflow of aqueous humor, the watery fluid that fills the anterior and posterior chambers of the eye. Aqueous humor is secreted by the ciliary processes of the eye, and passes through the posterior chamber and the pupil into the anterior chamber where it is reabsorbed into the venous system at the iridocorneal angle by way of the sinus venosus, also called the canal of Schlemm. Obstruction of aqueous humor outflow appears to be mainly responsible for elevated intraocular pressures.
The aim of current glaucoma treatment is the prevention of optic nerve damage and vision loss by lowering intraocular pressure. The search for better treatment regimens has moved back and forth between pharmaceutical and surgical methods as the first line of treatment.
In the United States, pharmaceuticals have been traditionally utilized as an initial response. Common pharmaceutical treatments for glaucoma include the systemic use of carbonic anhydrase inhibitors or topical applications of pilocarpine, timolol maleate, betaxolol HCl, levobunolol, metipranolol, epinephrine, dipivefrin, demecarium bromide, and echothiophate iodide. But, as is the case with most significant chemical therapies, the side effects of these medications may be severe while the treatment's efficacy is variable. Some of the drugs have unwanted systemic effects on cardiovascular functions and blood pressure, result in headaches or nausea, or cause ocular burning and irritation. However, the most frequent and perhaps most serious drawback to drug therapy is that patients, especially the elderly, often fail to correctly self-medicate. They forget to take their medication at the appropriate times or else administer the eyedrops improperly, resulting in under or over dosing. Because the effects of glaucoma are irreversible, when patients dose improperly--allowing ocular concentrations to drop below the appropriate therapeutic level--further permanent damage to vision occurs.
In Europe, where for many years there has been a strong focus on cost containment, surgery is the preferred approach. This is also the case in the United States when medication fails to control intraocular pressure or visual fields show progressive defects. It has been reported that in 1993 over 1,152,000 operations for regulating intraocular pressure were performed in the U.S. alone.
Glaucoma filtration surgery historically has been the most widely practiced procedure used in severe glaucoma cases. The fundamental principle of this surgery is to create an opening, or fistula, at the limbal region of the eye to facilitate the drainage of the aqueous humor, bypassing the pathological blockage in the anterior chamber angle. There are two basic approaches currently in use. In a full-thickness filtration procedure, a full-thickness sclerostomy is made, connecting the anterior chamber directly to the subconjunctival space. The main advantage of this procedure is the significantly lower intraocular pressures achieved postoperatively. However, because of its complications, this surgery is less frequently used than the second type of surgery, the trabeculectomy. In the trabeculectomy, a sclerostomy is performed under a scleral flap. This flap is then sutured back to its original bed in an attempt to minimize aqueous outflow runoff. The advantage of the trabeculectomy under the scleral flap is the tamponate effect provided by the resutured sclera causing a subsequent reduction of aqueous flow-through. Unfortunately, although this procedure provides short-term postoperative stability, final intraocular pressure levels are usually higher than those seen after full-thickness filtration, and the long term success rate is lower.
A major problem with both these approaches, and glaucoma filtration surgery in general, is the body's natural healing process. Glaucoma filtration surgery differs from most surgical procedures in that inhibition of wound healing is desirable to achieve surgical success. When normal wound healing occurs, filtration rates decrease and intraocular pressures rise, making necessary the inhibition of the healing response. Surgical failures occur most frequently due to an overwhelming wound healing response and scarring at the filtration site. Histological studies of human and lab animal surgeries suggest that failure of glaucoma filtration surgery is associated with the presence of dense fibrovasular connective tissue around the surgical site. This prevents diffusion of the aqueous humor from the subconjunctival space.
Since the turn of the century many efforts have been made to facilitate aqueous outflow and defeat the healing process by the insertion of devices in the surgical fistula. These devices have varied widely in size, material composition and design, one of the first being the horse hair implant of Rabbett and Moreau in 1906. Over time, aqueous shunts have become an increasingly popular and effective means of lowering intraocular pressure. Among the more recent devices, translimbal equatorial shunts have proven most effective. Examples of such devices include those disclosed in United States patents granted to Molteno (U.S. Pat. Nos. 4,457,757 and 4,750,901), Odrich (U.S. Pat. Nos. 5,041,081 and 5,127.901), and Baerveldt et al. (U.S. Pat. No. 5,178,604). Molteno's devices generally consist of ridged plates having drainage tubes for insertion into the anterior chamber of the eye. Odrich's patents disclose two ophthalmic implants for relieving pressure in the anterior chamber, both having one-way flow resisting valves, and residing under the conjunctiva. Baerveldt's apparatus comprises an elastomeric plate having a drainage tube tunneled through Tenon's capsule and the cornea and inserted into the anterior chamber. Virtually all conventional glaucoma aqueous drainage implants are designed to allow aqueous flow around their surface or to conduct the aqueous through a hollow tube either directly into a vein or bleb or to a large episcleral plate that will deposit aqueous in the posterior conjunctiva between the muscles.
Conventional aqueous shunts, however, are plagued by several peculiarities, including foreign body reactions and inflammation, as well as obstruction and infection. A major disadvantage of current open tube aqueous drainage devices is excessive aqueous drainage in the immediate postoperative period resulting in a flat anterior chamber and potential choroidal detachment. Profound hypotony, possibly leading to phthisis bulbi, is also a substantial risk. Excessive postoperative aqueous flow also causes expansion of the fibrous capsule beneath the rectus muscles of the eye. This mass effect stretches and tightens the muscles inducing heterotropia and motility restriction into the quadrant of the implant.
A mass effect also may be exerted simply by the bulky presence of the device itself beneath the muscle causing restriction of eye movement, scleral erosion, changes in eye curvature, or damage to adjacent vasculature and tissue. This is particularly true of rigid plastic or metal implants having valves. They tend to be large and complex in design. Other problems involve friction and wear imparted to the scleral flap by implanted devices, irritation of the iris endothelium caused by placement of implants into the anterior chamber, and aggravation produced by chronic forward and backward movement of the implants. Some glaucoma filtration surgeries also require the performance of peripherial iridotomies, wherein a transverse division of some of the fibers of the iris is performed to create a communication between the anterior chamber and the posterior chamber.
In spite of these shortcomings, aqueous drainage devices have been successful in many cases, but the operative procedure remains challenging and significant complications are not unusual.
It is thus an object of the present invention to provide a method and apparatus for lowering the intraocular pressure of an eye and mechanically inhibiting wound healing at the surgical site without producing a foreign body reaction, inflammation, obstruction, or infection.
Another object of the invention is to design and construct an intraocular implant that is pliable so as to fit the contours of the eye, that is soft enough to avoid scleral erosion, inducement of undesirable changes in eye curvature, or damage to adjacent vasculature and tissue, but that is resilient enough to maintain its shape and thickness. A like object is that the construction of the implant solves problems of friction and wear imparted to a scleral flap, irritation of the iris endothelium, and aggravation produced by implant movement.
A further object of the invention is to provide a device that naturally regulates the flow of aqueous humor by mimicking the trabecular meshwork, the device having a microstructure that allows drainage but never results in a post-surgical hypotony as seen with hollow tube devices.
Yet another object of the invention is to supply an intraocular implant that is small and simple, that has no valves, tubes or pressure sensing mechanisms, so as to avoid mechanical failure and allow for unobstructed eye movement.
A still further object of the invention is to furnish a method for surgically implanting an intraocular filtration device, which procedure is relatively simple and has few complications.